Precipitation hardening high strength cold rolled steel sheet and method for producing same

ABSTRACT

A precipitation hardening high strength cold rolled steel sheet comprising 0.03 to 0.25% C, not more than 1.5% Si, 0.6 to 2.5% Mn, 0.01 to 0.15% Al, 0.01 to 0.40% effective Ti (total Ti% 3.4(N%) - 1.5(0%) - 1.5 (S%)) and satisfying 4(C%) - 0.6% &lt; effective Ti% &lt; 4(C%), with the balance being Fe and unavoidable imparities.

United States Patent 1 Gondo et al.

[4 1 Dec. 31, 1974 PRECIPITATION HARDENING HIGH STRENGTH COLD ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME [75] Inventors: Hisashi Gondo; Hiroshi Takechi;

Tsuyoshi Kawano; Kazuo Namba, all of Kisarazu; Hiroaki Masni, Kimitsu; Koji Ozaki; Shunichi Uchida, both of Kisarazu; Koichi Sakurai, Kimitsu, all of Japan [73] Assignee: Nippon Steel Corporation, Tokyo,

Japan [22] Filed: July 9, 1973 [21] Appl. No.: 377,308

[30] Foreign Application Priority Data July 11, 1972 Japan 47-68670 June 20. 1973 Japan 48-69615 [52] US. Cl 148/12 F, 148/123 [51] Int. Cl C21d 7/14, C22c 41/02 [58] Field of Search 148/123, 12 F [56] References Cited UNITED STATES PATENTS 3,328,211 6/1967 Nakamura... 148/12 F 3,544,393 12/1970 Zanetti 148/12 F 3,625,780 12/1971 Bosch et al 148/12 F Primary Examiner-W. Stallard Attorney, Agent, or Firm-Toren, McGeady and Stanger [5 7 ABSTRACT 4 Claims, 1 Drawing Figure Falling Down of Sleel lngol ll O SpolWeld u Hi l Spot Weld PRECIPITATION HARDENING HIGH STRENGTH COLD ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME BACKGROUND OF THE INVENTION Recently, development of high strength cold rolled steel sheets having excellent press-forming property at high temperatures, tensile strength of50 to 70 Kg/mm yield strength of 40 to 65 Kglmm rapture strength of 15 to 30% and 'r'value of more than 1.1 has been demanded for automobile trims and skins. However, for obtaining high strength steel having tensile strength of 50 to 70 Kglmm addition elements required for strengthening the steel so highly are naturally limited, partially because commonly used strengthening elements such as C, Si and Mn can not be added unlimitedly due to the high carbon equivalent and in view of spot weldability. Then, addition of elements which form carbides and cause precipitation hardening is considered for the above purpose. As carbide-forming and precipitation hardening elements, Ti, Nb and V are widely known, but no teaching and guidance have been reported for the production of high strength cold rolled steel sheets having various properties and spot weldability required for press forming of automobile trims and skins. Most ofconventional commercially available cold rolled steel sheets for automobile are of soft steel grades having tensile strength of about 30 Kg/mm and the production technics for high strength cold rolled steel sheets have been left almost undeveloped. Particularly, the production of precipitation hardening high strength cold rolled steel sheets has been confronted with various difficulties and demand has been made for establishment of production technics through various new trials.

In this connection, the production technics for precipitation hardening high strength cold rolled steel sheets established by the present invention is novel and unique.

SUMMARY OF THE INVENTION The features of the present invention may be summarized as under.

I. A precipitation hardening high strength cold rolled steel sheet comprising 0.03 to 0.25% C, not more than 1.5% Si, 0.6 to 2.5% Mn, 0.01 to 0.15% Al, 0.01 to 0.40% effective Ti (total Ti% 3.4[N%] 1.5[%]- 1.5 [S%]) and satisfying 4 [C%] 0.6% effective Ti% 4 [C%], with the balance being Fe and unavoidable imparities. I 2. The steel slab having the composition as defined in the above (1) is heated at a temperature not lower than 1,200C, hot rolled, subjected to finishing rolling at a finishing temperature not lower than 870C, coiled at a temperature between 560 and 680C, acid pickled, cold rolled with a reduction rate not less than 30%, and subjected to recrystallization annealing at a temperature between 600 and 900C.

3. The recrystallization annealing temperature in the above (2) is higher than coiling temperature of the hot rolling.

The effective Ti content used in the present invention means Ti content obtained by reducing Ti amount which is combined with N, O, and S from the total Ti content.

The complexity in the production of precipitation hardening high strength cold rolled steel sheet is that it is necessary to obtain such a high strength as tensile strength of 50 to Kg/mm as well as high Tvalue in spite of the recrystallization softening annealing done after the cold rolling. In this point, one cannot find any technical guidance from the aforementioned Japanese Patent Publication Sho 47-4787.

The features of the present invention will be briefly explained hereinunder.

In order to obtain enough high strength properties and drawability as final cold rolled steel sheet, it is necessary to determine optimum conditions from view points of the amount and size of TiC precipitate. Thus, if almost all of the fine TiC effective for strengthening the steel is precipitated during the coiling of the hot rolling, or if the TiC is allowed to grow coarse, strength after the cold rolling and the recrystallization annealing is adversely affected, and it is very difficult to obtain high strength property as tensile strength of 50 to 70 Kg/mm On the other hand, when an appropriate amount of fine TiC is not allowed to precipitate in appropriate size, it is practically difficult to obtain the desired strength and drawability. The reasons are as follows. When the precipitation is done all at once at the recrystallization annealing, the precipitates of fine TiC effective for strengthening the steel are unstable and too much high strength or too much low strength results, and there exists not enough precipitate prior to the cold rolling favourable for the formation of a recrystallization texture effective for the drawability so that cold rolled steel sheets having good drawability cannot be obtained.

Thus, in order to obtain the desired strength and drawability, it has been found by the present inventors that it is most desirable to precipitate a part of the appropriate amount of fine TiC in appropriate size beforehand during the coiling of the hot rolling and precipitate the remainder ofthe fine TiC during the recrystallization annealing. And the following considerations are made for this procedure.

1. The finishing temperature range of the hot rolling and the coiling temperature range of the hot rollmg.

2. The reduction range of the cold rolling.

3. The combination of the recrystallization annealing temperature range with the coiling temperature range of the hot rolling.

4. The proper range of steel composition.'

When even one of the above factors is not satisfactory, it is difficult to produce the desired precipitation hardening high strength cold rolled steel sheets.

Regarding the finishing temperature of the hot rolling a higher temperature is desirable for the steel composition of the present invention. In case of conventional high strength hot rolled steel sheets, since the hot rolled steel sheet is the final product, rather a lower finishing temperature is applied in most cases to precipitate slightly coarse precipitates to improve toughness. However, in the present invention, as the cold rolled steel sheet is the final product, it should be avoided to precipitate coarse precipitates during the hot rolling prior to the completion of the finishing rolling, and thus a higher finishing temperature is more desirable. In this case, the problem of toughness can be solved by appropriate coarsening of the fine TiC during the recrystallization annealing.

Regarding the coiling temperature of the hot rolling, if the temperature is too high, almost all of TiC is precipitated or coarsened, so that the strength of the cold rolled steel sheets is not enough. On the other hand, if the coiling temperature is too low, the appropriate amount of fine TiC favourable for the strength and the drawability is not precipitated during the hot coiling, and a considerable amount of fine TiC precipitates during the recrystallization annealing to make the strength unstable, and particularly it is impossible to produce the cold rolled steel sheets having good drawability.

The reduction rate of the cold rolling is a veryimportant factor in the present invention. The optimum reduction rate of the cold rolling in the present invention is determined not merely for the aim to obtain a recrystallization texture favorable for the drawability as in the convention cold rolled steel sheets, but also in view of the fact that the amount of internal defects (dislocation) which varies depending on the reduction rate affects the amount and size of the TiC precipitates and the recrystallization temperature itself so that the optimum reduction rate of the cold rolling is determined thereby.

Regarding the recrystallization annealing temperature, it must be higher that the coiling temperature of the hot rolling. Otherwise it is impossible to produce the desired high strength cold rolled steel sheets of the present invention. The reason is that the desired strength of the final cold steel sheet product is not easy particularly in case of a precipitation hardening steel grade as mentioned above, and that in the present invention it is most desirable to precipitate beforehand a part of the fine TiC during the coiling of the hot rolling and to make it coarse appropriately and to precipitate the remainder of the fine TiC during the recrystallization annealing.

It is also clear that it is very useful to increase the r value for applications where drawability is of most concern and thus it is possible to remarkably expand the utility of the cold rolled steel sheets of the present invention.

Thus, in the present invention, the steel sheet after the recrystallization annealing is again cold rolled with a reduction rate not less than 30% and then subjected to continuous annealing between 600 and 900C, and the steel composition contains one or more elements from the group A consisting of Nb, V, Mo, and W in an amount of 0.01 to 0.1%, one or more from the group B consisting of Cr, Ni and Cu in an amount of 0.03 to 1.0%, and/or one or more from the group C consisting of Zr, Ca Mg and rare earth elements in an amount of 0.01 to 0.1% (amount to be added).

One of the features of the present invention is to obtain high strength properties and drawability due to the fine carbides such as TiC, and for this purpose it is necessary to control the precipitation of TiC etc. through the control of the hot and cold rolling conditions.

According to the conventional method for producing cold rolled steel sheets the conditions of the hot rolling and the cold rolling are precisely combined, whereas in the present invention as the cold rolling and annealing is done two times, it is necessary to set the lower limits both for the slab heating temperature and the finishing temperature of the hot rolling so as to dissolve fully Ti, C etc. and to set the upper limit for the coiling temperature so as to substantially restrict the precipitation of TiC etc. during the stage of the hot rolled steel sheet. Even in case of the steel composition according to the present invention, the optimum range of the coiling temperature for controlling the amount and size of the precipitates after the hot coiling would be narrow, if the one-time cold rolling and annealing method is applied as in the conventional art, but it is not necessary to set the lower limit severely in the present invention.

Advantages of the precipitates of TiC etc. due to the two-time cold rolling and annealing method according to the present invention are asunder.

By partially precipitating the fine TiC during the annealing after the first cold rolling and by allowing the fine TiC precipitates to grow to an appropriate size and precipitating the remainder of the fine TiC during the annealing after the second cold rolling, the desired strength and the recrystallization texture desirable for the drawability can be obtained. in this case, by the first cold rolling and annealing, the fine TiC favourable for the strength and drawability is effectively precipitated, and the fine TiC is precipitated more uniformly as compared with the conventional art in which the final product is obtianed by only one cold rolling and annealing after the hot coiling so that much higher strength and better drawability can be obtained finally by the second rolling and annealing. in this case the final annealing may be done either by a box annealing or by a continuous annealing, but it is desirable to employ continuous annealing in order to assure uniform annealing temperature. The conditions of the first cold rolling and annealing are limited naturally in view of the above objects. Thus it is necessary to set the lower limit of the cold reduction rate in order to precipitate the fine TiC uniformly during the first annealing. Also the temperature of the first annealing is important, and it is undesirable to apply a temperature below the hot coiling temperature.

For the second cold rolling and annealing, the lower limit of the cold reduction rate is important. The optimum cold reduction rate is determined not for the purpose of obtaining a recrystallization texture favourable for drawability as in case of conventional cold rolled steel sheets, but it is determined by the fact that the amount of internal defects which varies depending on the reduction rate affects the amount and size of the TiC precipitates'and also affects the recrystallization annealing temperature itself. Therefore, the lower limit of the reduction rate is essential also in the second cold rolling. Also the annealing temperature is important, and the lower limit of the annealing temperature is essential for precipitation of the fine TiC which has not been precipitated in the first cold rolling and annealing. However, it is not always necessary to apply a higher temperature than the first annealing temperature, because much internal defect is introduced again by the second cold rolling so that non-dissolved fine TiC can be precipitated. in this case, however, the upper limit of the annealing temperature is essential for maintaining the fine TiC required for the strength.

The reasons for the limitation of the chemical composition are as under.

Ti and C are essential elements for precipitation hardening of the steel. What is important in the present invention is that no satisfactory strength of the cold rolled steel sheet is obtained and the tensile strength is at most 50 Kg/mrn unless the condition of the effective Ti% 4 [C%]. And what is more important is that good spot weldability is required by the natures of the applications to which the present cold rolled steel sheet is intended. Even when the strength of the steel is high enough, safety of structures made of the steel sheet is not assured if the weld portion peels off. The present inventors have found that if the carbon content not fixed by Ti exceeds 0.15% the spot weldability lowers. Thus the condition of C% [effective Ti%] 0.15% must be satisfied. [effective Ti%] is considered to be approximately equal to the amount of carbon which combines with titanium, and C% [effective Ti%] is considered to be equal to the a yunt o f carbor 1 whic h i not fixed by titaniu n 1.

Si and Mn are also important for maintaining the strength.

S is not specifically limited in the present invention. In Ti-containing steels, S combines with Ti and sulfides in elongated form is hardly formed so that the amount of S is not required to be severely restricted as in ordinary steel compositions. Particularly in case of the present inventive cold rolled steel sheets, the sheet thickness is thin and no severe restriction of the S content is needed for the bendability of the sheet.

On the contrary, the maximum value of the Tvalue which indicates the drawability appears when S is present in the range of().0l2 to 0.02% in the present inventive steel sheet. The optimum content of S must be determined from the view point that the drawability is most important for the press-formability in case of cold rolled steel sheets as in the present invention.

As for the precipitation hardening elements, Nb, V, Mo and W other than Ti are effective, and addition of these elements in an appropriate amount further improves the high strength property and refines the carbides to improve the press-formability. Cr, Ni and Cu are effective for strengthening the steel, and promotes the high strength property, increases work hardenability to improve the balance between the strength and the ductility and improves the press-formability.

Zr, Ca, Mg and rare earth elements combine with S to spheroidize the sulfide inclusions and improves the impact properties and the press-formability.

Each minor element of the above groups added for improving the press-formability in a wide sense is useful for further improving the properties of the present inventive steel sheet and expanding the field to which the present inventive steel sheet can be applied.

At least 0.03% ofcarbon is necessary for maintaining the high strength property based on the fine TiC, but carbon contents more than 0.25% no substantial desirable effect on the strength is obtained, and on the contrary the toughness and weldability etc. are deteriorated. In the point of ductility not more than 0.15% C is desirable.

Si is effective to strengthen the steel and its content may be determined as case may require, but Si contents more than 1.5%, the nature of hot rolling scale is deteriorated and the temper colour is promoted during the annealing. And in order to obtain a good surface condition as in ordinary mild steel sheets, it is desirable to maintain the silicon content not more than 0.8%.

Mn is effective to strengthen the steel and for this purpose at least 0.6% Mn is necessary. On the other hand Mn contents more than 2.5%, the hardenability is promoted, and not only causes unstableness in the strength but also gives adverse effect on the weldability. From the point of strength, not less than 0.8% Mn is desired but from the point of drawability, not more than 1.8% Mn is desirable.

Al is effective for deoxidation of the steel and at least 0.01% Al is necessary. But when Al is contained in an amount more than 0.15%, the deoxidation effect saturates and problems of hot embrittlement due to AlN arise. From the point of ductility, not more than 0.1% A] is desirable.

Ti is essential for giving the high strength properties, and at least 0.01% effective Ti is required. But in case of a steel as in the present invention where the fine TiC and the precipitation hardening are the main object, even when the effective Ti is increased more than 0.40%, the strength does not substantially increase but on the contrary the embrittlement is promoted. Further from the point ofductility not more than 0.3% effective Ti is desirable.

Regarding the carbon content and the Ti content, it has been found by the present inventors that their relative proportions to other are very important as well as their respective contents. When the condition of [effective Ti%] /C% 4 is not satisfied, the amount of TiC which precipitates during the hot coiling and the recrystallization annealing is small, and thus it is difficult to obtain tensile strength of 50 to Kg/mm for the cold rolled steel sheets. When the strength properties are particularly of importance [effective Ti%]/C% 3.

The other determining factor is the spot weldability. As mentioned hereinbefore, when the amount of carbon which is not fixed by Ti is large, the spot weldability is poor, and thus the condition of C% Ar[effective Ti%] 0.15% must be satisfied.

Namely in order to satisfied both the strength requirement and the welding requirement, it is necessary to satisfy the condition of 4[C%]- 0.6% effective Ti% 4[C%].

Regarding Nb, V, Mo, and W, they play as precipitation hardening elements and effective to enhance the high strength properties and improve the pressformability by refining the carbides, and they must be present in an amount of at least 0.01%.

On the other hand, when they are present in an amount more than 0.1% their effects are saturated and the press-formability is lowered by the hardening of the steel.

Cr, Ni and Cu are effective to increase the high strength properties and improve the press-formability through improvement of balance between the strength and the ductility. For these purposes, it is necessary that they are present in an amount not less than 0.03%, preferably not less than 0,05%. But when they are present in an amount more than 1.0%, the press-formability is lowered by the hardening of the steel. Further in order to avoid the deterioration of the properties of the scale of the hot rolled sheets, it is desirable that they are present in an amount not more than 0.8%.

Zr, Ca, Mg, rare earth elements combine with S to spheroidize inclusions and improve the impact properties and the press-formability, and for this effect, at least 0.01% (amount to be added)is necessary. On the other hand, when they are present more than 0.1% their effects are staurated and oxide inclusions are formed and the bending properties are lowered. Thus it is desirable that they are present not more than 0.06%.

When P is added for improvement ofr'value it is added in an amount more than 0.03%. However, in view of the secondary work cracking, P/C 2.0 is desired.

Explanations will be made on the reasons of the limitations of the production process.

Unless the heating temperature is more than 1,200C, it is impossible to dissolve fully Ti and C in the stage of the hot slab, and is impossible to maintain the strength of the final product. In case when the strength is of primary importance, the condition of 4[C%]- 0.6 effective Ti% 3[C%] is desirable.

Regarding the finishing temperature, it is necessary to maintain the temperature not lower than 870C to avoid precipitation of TiC during the hot rolling prior to the finishing rolling in order to assure the high temperature strength properties of the final cold rolled steel sheets. In case where the strength is particularly important, a finishing temperature not lower than 890C is desirable. And in view of the coiling temperature, a finishing temperature not higher than 960C is desirable.

Regarding the hot coiling temperature, when the temperature is too low, the fine TiC favourable for the strength and drawability of the steel does not fully precipitate during the hot coiling so that a considerable amount of TiC is precipitated during the recrystallization annealing, thus causing unstable strength and difficulty in obtaining the desired strength, and at the same time there exists not enough precipitate for forming a texture favourable for drawability so that only low "r'value is obtained. Thus, the lower limit of the coiling temperature is set at 560C, but in view of the finishing temperature, a hot coiling temperature higher than 570C is desirable. On the other hand, with a high temperature coiling higher than 680C, most of TiC including TiC which was partially coarsed during the hot coiling precipitates so that it is very difficult to obtain the high strength properties of the cold rolled steel sheets. In case where the high strength properties are particularly important, a hot coiling temperature not higher than 650C is desirable.

Regarding the cold reduction rate, with a reduction rate less than 30%, the amount of internal defects such as dislocation is irregular, and it is impossible to produce a uniform precipitation site of TiC in the grains, and the strain energy stored by the cold rolling is too small for recrystallization so that the temperature zone for the partial recrystallization and the optimum temperature zone for the precipitation hardening overlap, thus causing difficulties in production. In case where drawability is particularly important, a reduction rate not less than 40% is desirable.

Regarding the recrystallization temperature, a satisfactory recrystallization temperature below 600C a full recrystallization can not be attained. On the other hand, with a recrystallization temperature higher than 900C, the structure partially transform into austenite, thus causing large unstability in the strength. In order to partially precipitate the fine TiC which did not precipitate during the hot coiling, it is necessary that the temperature of the first recrystallization annealing is higher than the hot coiling temperature. The second annealing not lower than 600C is necessary in order that part of the fine TiC which precipitated during the first annealing is allowed to grow appropriately, and part of the TiC which did not precipitate during the first annealing is allowed to precipitate finely. In view of the continuous annealing, an annealing temperature not lower than 650C is desirable in order to promote the recrystallization.

On the other hand, with an annealing temperature higher than 900C, a partial transformation into austenite is caused so that great unstability takes place in regard to the strength of the steel.

The present invention will be more clearly understood from the following examples.

EXAMPLE 1:

The steel was prepared in a converter and made into slabs by an ordinary ingot-making method and partly by a continuous casting (B,, B C and C and subjected to hot rolling, cold rolling and annealing as specified in Table l to obtain cold rolled steel sheets of final thickness of 1.0mm and 1.6mm. The sheets were all subjected to 1.0% skin pass rolling. The chemical compositions of the steels as well as their mechanical properties and weldability are shown in Table l. The box annealing in this example was conducted by heating'the steel at a heating rate 20 to 40C/hour, holding the steels at different temperatures for 6 hours and cooling the steels in the furnace. On the other hand, the continuous annealing was done by rapidly heating the steels, holding them at their respective temperature for 2 to 4 minutes and subjecting them at a temperature between 400 and 450C except the grades C and C The testing method for the weldability is as under. The samples were spot welded under the following conditions and subjected to the under two tests.

According to the first test, samples showing Vickers hardness value (10 Kg load) exceeding 400 at and near the spot weld were estimated as bad (in Table l O shows good and X shows bad).

According to the second test, a structure prepared by spot welding as shown in FIG. 1, was cooled to 30C, and a steel lamp of 80 kg load was dropped on the structure to see if there is peeling off at the spot weld, and a sample showing peeling off at even one point was estimated as bad (X mark), and s sample showing completely no peeling off was estimated as good (0 mark).

As shown by table 2, the cold rolled steel sheets of the chemical composition produced according to the present invention show both excellent drawability and weldability, with a tensile strength of 50 to Kg/mm What should be noticed in this point is that in order to satisfy both the strength properties and the spot weldability, the steel composition limited by the condition of 4[C%]- 0.6% effective Ti% 4[C%] is necessary, and this limitation is very important for the present high strength cold rolled steel sheets to be used in applications where safety of structures is most con cerned.

Another point to be noticed is that it is possible to obtain satisfactory steel sheets even without an overageing treatment, and thus remarkable advantage in production is attained as compared with conventional high strength cold rolled steel sheets.

EXAMPLE 2 The steels of compositions shown in Table 2 were prepared in a converter, and subjected to an ordinary ingot-making method and partly to continuous casting (F, F to obtain slabs, which were subjected to hot rolling, cold rolling and annealing as specified in Table 1 to obtain cold rolled steel sheets of final thickness of 1.0mm and 1.6mm. The sheets were all subjected to 1.0% skin pass rolling. The mechanical properties Tvalues and weldability of the obtained cold rolled steel sheets are shown in Table 2. The box annealing in this example was conducted by heating the steels at a heating rate 20 to 40C/hour, holding them at different temperatures for 6 hours and subjecting them to an ameter, and the bore was expanded using a conical punch. The estimation was done by the percent of a maximum diameter of the bore expanded until a crack took place around the bore to the original bore diameo over-ageing treatment at a temperature between 400 5 ter. A larger value shows better local deformatlon abiland 450C excepting the grades F and F The estlma- 1ty. non and testing method for weldability are same as 1n Example 1, and results are shown in Table 2. As understood from the results shown in Table 1 the Further, stamping and bore expansion tests were concold rolled steel sheets according to the present mvenducted as under to estimate local deformation ability l0 tron show excellent drawabillty, stamping and bore ex among the press-formabillty. pansion property and weldability, with a tensile A dlSC sheet was stamped to give a bore of 20mm distrength of 50 to 70 Kg/mm TABLE 1 Heating Finishing Coiling Reduction Chemical compositlon (percent) temperatemperatemperarate oicold 'Ihlek- 4[C%] ture ture ture rolling ness C S1 Mn P S Al T1 4[C%] 0.6% C.) C.) C.) (percent) Spot; weldability Annealing Tensile Low temtemperastrength Yield point Elongation Hardness perature Grade Annealing ture C.) (kg/mm?) (kg/nun?) (percent) 7 Value In weld peellng test 131* Box annealing 680 53.2 41.0 29.6 1.32 O 0 do 030 47.0 35.4 34.1 1.24 0 o 030 57.5 45.1 20.1 1.33 o o 030 43.3 32.0 40.1 1.13 o o 000 53.4 40.0 27.0 1.37 o o 090 40.5 30.0 34.0 1. 24 o 0 500 32.0 30.3 0.4 1.01 o 0 030 05.0 53.4 23.2 1.53 o o 000 33.5 10.7 43.4 1.03 o o 030 41.2 20.0 40.3 1.30 0 o 530 30.4 37.0 4.2 0.03 o o 000 07.2 55.0 21.0 1. 42 o o 530 02.4 01.0 2.3 1.02 o o 040 75.4 03.1 10.2 1.23 o o 750 01.2 40.0 20.3 1.74 o o 090 03.4 53.2 20.3 1.31 o 0 000 70.0 53.4 13.3 1.34 o o 000 50.4 47.4 24.0 1. 24 x X 000 02.3 50.0 24.2 1.45 o 0 000 54.4 42.3 23.3 1.33 o X 700 70.3 03.2 10.1 1.33 o 0 700 00.0 57.3 21.2 1.00 X X 030 53.0 41.3 20.1 1.05 o o 030 42.4 30.4 30.2 1.10 o o 030 70.0 00.2 10.3 0. 03 o 0 Cl Continuous annealing" 770 55. 4 43.4 28.6 1. 43 O 0 02* .410 300 54.3 42.4 23.0 1.44 o o 530 30.3 33.4 3.0 0. 00 o o 350 53.3 40.0 200 1.30 o H o 7 Spot weldability Low tem- Hardness perature r Value in weld peeling test Annealing Tensile temperastrength Yield point Elongation ture C.) (kg/mm!) (kg/mm!) (percent) Annealing Grade 00X000X00oo000 IILLLLLLLLLQLL 2 0 I Z l Chemical composition (percent) Group A Group B Group C TABLE 2 =Bad.

O Good; X

Plate thickness (mm.)

Inventive steels.

IILLLOWQL Second cold rolling annealing Annealing Cold Annealing temperareduction tem erw ture( C.) rate(perecnt) ture( G.)

Stamping Spot weldability bore expansion Low temtest Hardness perature 1 Value (percent) in weld peeling test First cold rolling annealing Cold reduction e(percent) Annealing type 55 Box annealing- 70 Box annealing 70 Continuous anneal Yield Elongapoint tion (kg/mm!) (percent) Coiling temperatemperae( C.) ture( C.) rat Tensile strength (kg/mm!) Heating Finishing temperature( O.) tur Grade OOOOXOOOOOXOOOOOOO mmnw z uwwmmwwwwmwwwmm LLLLLLLLLLLLLLLLLL 64864246363626.4464 ZRWnmQMOMQQWRWQLQmQWDWiZZZ- 445545445552333446 .mwLLQQmOMQLLQLTAXmAmO 55 766556674444557 Inventive steel.

What is claimed is:

l. A method for producing a precipitation hardening high strength cold rolled steel sheet comprising heating a steel slab containing 0.03 to 0.25% C, not more than 1.5% Si, 0.6 to 2.5% Mn, 0.01 to 0.15% Al, 0.01 to 0.40% effective Ti (total [Ti%]- 3.4[N%]- 1.5[O%]- 1.5[S%]) and satisfying the condition of 4[C%]- 0.6% effective Ti% 4[C%], with the balance being Fe and unavoidable impurities at a temperature not lower than 1,200C, hot rolling the slab, subjecting the hot rolled sheet to a finishing rolling at a temperature not lower than 870C, coiling the sheet at a temperature between 560 and 680C, acid pickling the sheet, cold rolling the sheet with a reduction rate not less than and subjecting the sheet to recrystallization annealing at a temperature between 600 and 900C.

2. A process according to claim 1 in which the temperature of the recrystallization annealing is higher than the hot coiling temperature.

3. A process according to claim 1 in which the sheet after the recrystallization annealing is again cold rolled with a reduction rate not less than 30% and subjected to recrystallization annealing at a temperature between 600 and 900C.

4. A method as claimed in claim 1, wherein said steel slab further comprises at least one element selected from at least one of Groups A, B and C in a total amount of 0.01 to 0.1% for Group A elements, 0.03 to 1.0% for Group B elements and 0.01 to 0.1% of actually added amount for Group C elements, said Group A consisting of Nb, V, Mo and W, said Group B consisting of Cr, Ni and Cu, and said Group C consisting of Zr, Ca, Mg and rare earth elements. 

1. A METHOD FOR PRODUCING A PRECIPITATION HARDENING HIGH STRENGTH COLD ROLLED STEEL SHEET COMPRISING HEATING A STEEL SLAB CONTAINING 0.03 TO 0.25% C, NOT MORE THAN 1.5% SI, 0.6 TO 2.5% MN, 0.01 TO 0.15% AL, 0.01 TO 0.40% EFFECTIVE TI (TOTAL (TI%)-34(N%)-1.5((0%)-1.5(S%) AND SATISFYING THE CONDITION OF F(C%)-0.6%<EFFECTIVE TI% <4(C%), WITH BALANCE BEING FE AND UNAVOIDABLE IMPURITIES AT A TEMPERATURE NOT LOWER THAN 1, 2::=C, HOT ROLLING THE SLAB, SUBJECTING THE HOT ROLLED SHEET TO A FINISHING ROLLING AT A TEMPRATURE NOT LOWER THAN 870*C, COILING THE SHEET, COLD ROLLING THE SHEET WITH AND 680*C, ACID PICKLING THE SHEET, COLD ROOLING THE SHEET WITH A REDUCTION RATE NOT LESS THAN 30% AND SUBJECTING THE SHEET TO RECRYSTALLIZATION ANNEALING AT A TEMPERATURE BETWEEN 600* AND 900*C.
 2. A process according to claim 1 in which the temperature of the recrystallization annealing is higher than the hot coiling temperature.
 3. A process according to claim 1 in which the sheet after the recrystallization annealing is again cold rolled with a reduction rate not less than 30% and subjected to recrystallization annealing at a temperature between 600* and 900*C.
 4. A method as claimed in claim 1, wherein said steel slab further comprises at least one element selected from at least one of Groups A, B and C in a total amount of 0.01 to 0.1% for Group A elements, 0.03 to 1.0% for Group B elements and 0.01 to 0.1% of actually added amount for Group C elements, said Group A consisting of Nb, V, Mo and W, said Group B consisting of Cr, Ni and Cu, and said Group C consisting of Zr, Ca, Mg and rare earth elements. 